1. Field of the Invention
The present invention relates to a read head with leads to shields shorts for permitting a thinner second read gap layer and improving read signal symmetry and, more particularly, to such a read head wherein the first and second shield layers are extensions of the first and second lead layers.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinning layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from muinimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.             V      1        -          V      2            max    ⁢          xe2x80x83        ⁢          (                        V          1                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  V          2                    )      
For example, +10% readback asymmetry means that the positive readback signal VI is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In these applications +10% readback asymmetry can saturate the free layer in the positive direction and will reduce the negative readback signal by 10%. An even more subtle problem is that readback asymmetry impacts the magnetic stability of the free layer. Magnetic instability of the free layer means that the applied signal has disturbed the arrangement or multiplied one or more magnetic domains of the free layer. This instability changes the magnetic properties of the free layer which, in turn, changes the readback signal. The magnetic instability of the free layer can be expressed as a percentage increase or decrease in instability of the free layer depending upon the percentage of the increase or decrease of the asymmetry of the readback signal. Standard deviation of the magnetic instability can be calculated from magnetic instability variations corresponding to multiple tests of the free layer at a given readback asymmetry. There is approximately a 0.2% decrease in standard deviation of the magnetic instability of the free layer for a 1% decrease in readback asymmetry. This relationship is substantially linear which will result in a 2.0% reduction in the standard deviation when the readback asymmetry is reduced from +10% to zero. The magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative.
The location of the transfer curve relative to the bias point is influenced by three major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demagnetizing (demag) field HD from the pinned layer, and a net sense current field HI from all conductive layers of the spin valve except the free layer. The strongest of these forces is the net sense current field HI from the conductive layers of the spin valve sensor. In a bottom spin valve sensor where the free layer structure is closer to the second shield layer than to the first shield layer the majority of the conductive layers is located between the free layer structure and the first shield layer. The only conductive layer between the free layer structure and the second shield layer is a cap layer typically constructed of tantalum (Ta) which has a high resistance to the sense current. Accordingly, when the sense current is conducted through the spin valve sensor the net sense current field HI acting on the free layer structure is due to the sense current fields caused by the conductive layers between the free layer structure and the first shield layer minus the small sense current field due to the cap layer. The difference is the net sense current field which, as stated hereinabove, is the largest field acting on the free layer structure urging the magnetic moment of the free layer structure to be positioned at some angle to a zero bias position which is parallel to the ABS.
The sense current field needs to be counterbalanced so that the magnetic moment of the free layer will remain parallel to the ABS when the read head is in the quiescent condition. The forces available for counterbalancing are the aforementioned net demag field HD and the ferromagnetic coupling field HFC. The net demag field HD depends upon the type of pinned layer structure employed in the spin valve sensor. If the pinned layer structure is a single ferromagnetic layer composed of one or more ferromagnetic films the demag field HD is higher than when an antiparallel (AP) pinned layer structure is employed. Accordingly, the single pinned layer would be advantageous for providing a greater demag field HD for counterbalancing the net sense current field HI. However, the AP pinned layer structure is more desirable for a spin valve sensor than the single pinned layer since the AP pinned layer structure has improved thermal stability, that is, its magnetic moment retains a pinned direction at higher temperatures and fields than the single pinned layer. The AP pinned layer structure includes an antiparallel coupling layer which is located between ferromagnetic first and second AP pinned layers. Since there is partial flux closure between the first and second AP pinned layers the net demag field HD is considerably less than a single pinned layer. This causes a greater exchange coupling between the first AP pinned layer and the pinning layer for promoting the aforementioned thermal stability. Accordingly, it would be desirable to employ the AP pinned layer structure in the spin valve sensor even though its effect of counterbalancing the net sense current field HI is less than the single pinned layer. Typically, the ferromagnetic coupling field HFC is antiparallel to the net demag field HD which means that the ferromagnetic coupling field HFC is additive with the sense current field HI. This unfortunately increases read signal asymmetry. Accordingly, there is a need for providing a read head wherein read signal asymmetry can be lessened even though an AP pinned layer structure is employed.
The present invention provides a read head with a first lead layer from a spin valve sensor shorted to the first shield layer and a second lead layer from the spin valve sensor shorted to the second shield layer so that the first and second shield layers function as lead extensions for the first and second lead layers to terminals of the read head. In this manner the first and second lead layers can be significantly thinner than prior art first and second lead layers since the first and second lead layers can extend for a relatively short distance before being shorted to the first and second shield layers so that the first and second shield layers provide a large expanse of conductive material to carry the sense current to and from the read head terminals. When the first and second lead layers are thinner this causes each of the first and second lead layers to have a smaller step or rise as it extends from the spin valve sensor. Accordingly, with smaller steps the second read gap layer has less likelihood of having pin holes where it covers the steps. This means that the second read gap layer can be thinner than previous second read gap layers and still provide adequate coverage and insulation over the steps of the first and second lead layers without developing pin holes which, in turn, cause shorts between the lead layers and the shields. In the present invention, however, coverage of the step of only one of the lead layers is necessary since the other lead layer is shorted to the second shield layer.
The thinner second read gap layer performs three important functions, namely: (1) promotes read signal symmetry; (2) improves heat dissipation between the lead layers and the shield layers; and (3) promotes linear read density. Because of the conduction of the sense current IS through the spin valve sensor each of the first and second shield layers produces an image current field HIM which is exerted on the free layer structure. Since the free layer structure in a bottom spin valve sensor is located closer to the second shield layer than to the first shield layer, there is a net image current field HIM which can be employed for counterbalancing the sense current field HI. However, when the second read gap layer is made thinner this places the free layer structure even closer to the second shield layer which will increase the net image current field HIM for still further counterbalancing the net sense current field HI on the free layer structure. The second advantage occurs because the lead layers are directly connected to the shield layers so that the shield layers function as heat sinks for the first and second lead layers. Further, since the second read gap layer is thinner there is still further heat dissipation between the first lead layer, which is connected to the first shield layer, and the second shield layer. In regard to the third advantage, the thinner second read gap layer decreases the read gap which is measured between the first and second shield layers so that the read head is capable of writing more bits per linear inch along a track of a rotating magnetic disk. In a preferred embodiment the invention employs a pinning layer which is made of platinum manganese (PtMn) which provides a negative ferromagnetic coupling field xe2x88x92HFC which is parallel to the net demag field HD and parallel to the net imaging field HIM so that the sense current field is counterbalanced by three fields, namely, net demag field HD, net imaging field HIM and ferromagnetic coupling field HFC.
An object of the present invention is to provide a read head wherein a net sense current field HI acting on a free layer structure of a spin valve sensor can be more adequately counterbalanced by other magnetic fields for promoting read signal symmetry.
Another object is to accomplish the previous object with an antiparallel (AP) pinned layer type spin valve sensor.
A further object is to provide the foregoing objects along with improved heat dissipation from first and second lead layers of the read head.
Still another object is to accomplish the foregoing objects along with increasing linear read density of the read head.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.